memctx: add _manifest implementation that computes the filenode This is an initial implementation of having a manifest for memctx that computes the hash in the same way that filenodes are computed elsewhere.

basectx: pass raw context objects to patch.diff

patch: use ctx.node() instead of bare node variable Future patches will allow patch.diff to take a basectx so node1 (and node2) could make hexfunc error out. Instead, we'll call the node function on the context object directly.